A lithium ion secondary battery having a high energy density is recently in wide use as a power source for compact portable electronic appliances such as a mobile phone, a note-type personal computer and the like.
Such a lithium ion secondary battery is manufactured through processes of layering or rolling a cathode and an anode in the form of sheet and, for example, a porous resin film together, placing the resulting layered or rolled body in a battery container formed of, for example, a metal can, pouring an electrolytic solution into the battery container, and air-tightly closing and sealing the container. However, in recent years, it is strongly demanded to make such compact portable electronic appliances as mentioned above further small and lightweight. Accordingly, it is also demanded to make a lithium ion secondary battery further thin and lightweight, and a laminate film battery container has come to be employed in place of a conventional metal can container.
Many studies and inventions for providing high capacity and high output lithium ion secondary batteries have also been made on materials for a cathode and an anode. However, in spite of such energetic studies and inventions, there still remains a problem that when a lithium ion secondary battery is repeatedly charged and discharges at ordinary temperature or under an atmosphere at elevated temperature, the capacity of the battery falls, output characteristic deteriorates, and safety reduces.
As the reasons of the above-mentioned problems, there may be mentioned, for example, that lithium deposits on an anode when a battery is charged; metal ions are dissolved in an electrolytic solution from a cathode active material; an organic solvent is oxidized on a cathode to generate cation radicals, which are then reduced on the surface of anode; metal ions are dissolved in an electrolytic solution from a cathode active material and a collector and deposit on the surface of anode; an electrolytic solution is decomposed on the surface of electrodes to generate lithium fluoride, etc. to decrease the amount of lithium ions which contribute to capacity, and as a result, irreversible capacity increases (Pankaj Arora, Ralph E. White, Marc Doyle, Capacity Fade Mechanisms and Side Reactions in Lithium-Ion Batteries”, Journal of Electrochemical Society, Vol. 145, No. 10, October 1998).
Under these circumstances, there is strong demand for lithium ion secondary batteries which have high safety. And, for example, it has been proposed to use lithium manganate or lithium iron phosphate as a cathode active material because it is excellent in stability at an elevated temperature. However, lithium manganate has a problem in cycle characteristic at an elevated temperature, while lithium iron phosphate has a problem when power output is to be increased. Thus, neither of the compounds has been put to practical use to a substantial extent up to present.
It is believed that the reason why the cycle characteristic of battery deteriorates at an elevated temperature when lithium manganate is used in a cathode is that manganese in a cathode active material dissolves in an electrolytic solution as ions, and the dissolved manganese ions are reduced and deposited on an anode.
Then, it has been proposed, for example, to use a separator comprising a porous polyethylene resin film and cation exchange resin added thereto in a nonaqueous electrolyte secondary battery using a spinel structure lithium manganese composite oxide in a cathode in order to solve the problems mentioned above (JP 2000-21381A). However, the inventors have found that there is still a problem in such a nonaqueous electrolyte secondary battery that since the proportion of cation exchanging functional groups exposed to the surface of separator is small, metal ions of manganese, etc. are not captured efficiently, and on the other hand, the strength of separator is reduced.
It has been also proposed to use a separator of which surface is modified by cation exchanging groups in a nonaqueous electrolyte secondary battery comprising a manganese-containing composite oxide as active material (JP 2002-25527A). However, the inventors have found that there is still a problem in this case that since the diameter of ions of a metal such as manganese is in general larger than the diameter of pores which a porous membrane used as a separator has, the probability that cation exchange groups encounter metal ions is low so that the metal ions are not captured efficiently, and hence there is a problem in cycle characteristic of the resulting battery at an elevated temperature.
It has been further proposed that in a nonaqueous electrolyte secondary battery comprising a cathode, an anode, a separator and an electrolyte, a chelating agent is contained in at least one of the anode, the separator and the electrolyte (JP 2004-63123A). However, the inventors have found that there is still a problem in this case that the chelating agent causes undesirable side reactions on the cathode and the anode due to oxidation-reduction reactions thereof thereby to deteriorate the battery performance.
Furthermore, it has been proposed that in a nonaqueous electrolyte secondary battery provided with a cathode, an anode, a separator and an electrolyte, a chelate forming polymer is contained in at least one of the cathode, the anode and the separator (JP 11-121012A). However, the inventors have found that there is still a problem in such a battery that the amount of the chelate forming functional groups contained in the battery is small, and accordingly metal ions are not captured efficiently. In addition, the chelate forming polymer has no direct participation in charge and discharge of a nonaqueous secondary battery, and as a consequence, when the chelate forming polymer is contained in the cathode or the anode, the resulting battery has a reduced initial capacity.